Update "gdb --configuration"

[Eli Zaretskii]

> Date: Fri, 9 Mar 2018 11:57:52 +0100
> From: Joel Brobecker <brobecker@adacore.com>
> Cc: gdb-patches@sourceware.org
> 
> Looks good to me, modulo one tiny think-o below:
> 
> > +#if HAVE_LIBMPFR
> > +    fprintf_filtered (stream, _("\
> > +             --with-libmpfr\n\
> 
> It's actually --with-mpfr.
> 
> > +"));
> > +#else
> > +    fprintf_filtered (stream, _("\
> > +             --without-libmpfr\n\
> 
> Same here.

Thanks, fixed and pushed to the master branch.